Long life incandescent switching system

ABSTRACT

A supply and switching system for incandescent indicator lights is disclosed. A solid-state AC switch is used in conjunction with a current-limiting-transformer supply. The use of the current-limiting transformer protects the solid-state switch from the large current transients that are sometimes caused by failures in incandescent bulbs.

BACKGROUND OF THE INVENTION

A high level of reliability is required of indicator lights that areused in nuclear-reactor control panels as well as in many other types ofequipment. The extent to which this requirement is fulfilled is affectednot only by the bulb itself but also by its electrical environment.Accordingly, the designer should give serious attention to the design ofthe switching and supply systems that constitute the electricalenvironment of the bulb.

Among the types of switching and supply systems currently used is thecombination of a transformer with a mechanical relay or switch. Thetransformer supplies AC power, and the relay or switch controlsapplication of the power to the bulb. This combination is simple, andits use of an AC source makes it more merciful to the bulb filament thanit would be if it employed a direct-current source. The mechanical relayor switch, however, is relatively large and unreliable in comparisonwith the solid-state switch. In contrast to a solid-state switch,moreover, a mechanical switch or relay cannot be activated at zerocrossings of the AC supply, and this results in an unfavorable effect onbulb longevity.

The solid-state switch also has disadvantages. Solid-state switches failwhen subjected to currents that exceed their rated capacities, and suchcurrents may be drawn by incandescent bulbs when they fail short, asthey occasionally do. The shorted bulb draws a large current whosemagnitude is limited only by line impedances. Therefore, if asolid-state switch has been chosen that has a continuous-current ratingnot greatly in excess of the normal bulb current, its short-durationcurrent rating may well be exceeded by the current drawn by the shortedbulb. The short-duration current rating of the solid-state switch canalso be exceeded when a cold bulb is turned on. Accordingly, in order toavoid damage to the solid-state switch, the designer must chooseswitches with continuous-current ratings considerably in excess of thenormal bulb current. This, of course, increases expense and reduces theadvantages that solid-state switches would otherwise have overmechanical switches and relays.

In order to avoid choosing a solid-state switch with acontinuous-current rating greatly in excess of the normal currentrequirement of the bulb that it controls, designers have protected thesolid-state switch by providing a fuse in series with it. Fuses provideonly limited protection, however, since they cannot always be relied onto blow before the switch has been damaged. In addition, the use of afuse makes it necessary to replace both the fuse and the bulb if thebulb fails short. Furthermore, when several indicator lights arecontrolled by the same switch, the use of fuse makes it necessary tochange the fuse before the defective bulb can be located.

A supply and switching system that protects solid-state switches withoutusing fuses is the combination of a solid-state switch with acurrent-limited DC power supply. Though simple in concept, this approachgreatly increases the cost in systems in which large numbers of bulbsare to be powered. In addition, this type of system exhibits theunfavorable bulb-longevity characteristics of a DC supply.

It is apparent that the present state of this widely used and wellexplored art requires the designer to make trade-offs among bulblongevity, circuit reliability, and expense because the art has providedno means for obtaining the optimum in all three.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is a switching andsupply system that affords increased bulb life, allows the use ofsolid-state switches, and minimizes supply expense, thereby eliminatingthe trade-off requirement that has heretofore plagued the art.

According to the present invention a supply and switching system isprovided wherein a current-limiting transformer is used to supply powerto an incandescent bulb, and the power is controlled by means of asolid-state switch. The current-limiting transformer permits the use ofan alternating-current power source and a low-current solid-stateswitch, thus allowing the cost of a complicated power supply to beavoided while providing the longevity benefits of AC power.

BRIEF DESCRIPTION OF THE DRAWING

These and further features and advantages of the invention becomeevident in the description of the embodiment shown in the attacheddrawing, which is a schematic diagram of the preferred embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The FIGURE illustrates a switching and supply circuit in which bulbs 30are to be supplied power by current-limiting transformer 10 throughswitching means 35. Bulbs 30 are illustrated as a plurality of bulbs inparallel, but the invention applies equally to a single bulb. Aplurality is shown only because this is typical in the redundant systemsoften required in nuclear-reactor applications. In fact, the load neednot be an incandescent bulb or bulbs; any type of load that is subjectto current surges is contemplated by the present invention.

Transformer 10 is illustrated as having two cores 12 and 16, each beingmagnetically coupled to the primary windings of transformer 10. Core 12is shown as being coupled to secondary windings 14 and 18, while core 16is shown coupled to windings 20 and 22. This is thought to be asatisfactory schematic representation of the current-limitingtransformer of the preferred embodiment, which has a single primary coresection connected magnetically across two separate secondary coresections. It would be more customary to have a single core symbol forsuch a transformer, but it is important for present purposes toemphasize the independent functioning of the two core sections.

Transformer 10 is a magnetic shunt transformer, a type of transformerthat limits the amount of current induced in one or more of itssecondaries. An alternate magnetic path is provided in parallel witheach of the secondary sections. When a secondary section saturates, asit is designed to do at a predetermined current level, the associatedalternate path diverts any additional flux away from the saturatedsecondary. As can be appreciated, if one secondary section is saturatedwhile the other is not, the current-limiting action will only take placein one secondary section. This is why the secondary sections are shownas being parts of different cores.

Secondary windings 14 are connected at one end to threeparallel-connected incandescent bulbs and at the other to solid-stateswitch 35, which completes the bulb circuit. Solid-state switch 35 canbe any one of the many types commercially available. A typicalsolid-state switch might provide opto-isolation by having the inputcircuit, terminals 40 and 42 and LED 38, connected only optically to therest of the switch. Element 36 represents a gating circuit sensitive tolight. It gates triac 34 on only when LED 38 is forward biased enough toprovide the light required by gating circuit 36. In the preferredembodiment, gating circuit 36 would trigger the triac only at zerocrossings; as mentioned before, this contributes to bulb life. A snubbercircuit is suggested by the combination 32 of a series capacitor andresistor in parallel with the triac. This again would be part of thetypical solid-state switch package, and its purpose would be to preventdv/dt triggering of triac 34. As is apparent, the solid-state switch 35is merely exemplary; a wide variety of solid-state switches arecommercially available, and ordinary design considerations woulddetermine which type to use.

While it is possible to provide more than two secondary core sections ona magnetic-shunt transformer, only two are shown, since that is thetypical number. Windings 18, 20 and 22 are connected to loads not shown.To take advantage of the characteristics of the present invention, theseloads would typically be similar to bulbs 30 in that they would drawrelatively small currents during most of their operation but would besubject to large-current surges under some conditions.

In operation, AC voltage is applied at terminals 11 and 23, causing ACvoltages to be present across windings 14, 18, 20 and 22. If bulbs 30are to be lighted, voltage is to be applied to terminals 40 and 42,which causes LED 38 to emit light. This enables gating circuit 36 totrigger triac 34 at zero crossings of the voltage across windings 14. Ifthe filaments are cold, bulbs 30 may momentarily tend to draw more thanthe instantaneous-current rating of switch 35. If this is the case, thesecondary core section on which windings 14 and 18 are wound willsaturate, allowing the associated alternate flux path to divert fluxaway from the secondary core section associated with windings 14 and 18.The total current in windings 14 and 18 is thereby kept below apredetermined maximum. As the filaments warm up, they tend to draw lesscurrent, so the core returns from saturation, and ordinary operation isresumed.

It is to be noted that the current limiting involves a reduction involtage at the output terminals of the affected secondaries. Thus, aheavy load on secondary 14 will cause transformer 10 to reduce thevoltage across windings 18. Windings 20 and 22, on the other hand, willnot be affected. Of course, if it is desired that no loads be unaffectedby current limitations on any of the other loads, the designer will wantto keep each winding on a separate core section. If the interferencewith one load by another load is tolerable, however, the embodimentshown in the FIGURE may effect a cost savings.

As is evident from the foregoing description, it is possible, throughthe use of an embodiment of the present invention, to take advantage ofthe convenience and zero-crossing switching ability of the solid-stateswitch on loads subject to high-current surges without requiring theswitch to withstand the surge currents and without employing expensiveDC supplies.

What is claimed is:
 1. A switching and supply circuit for supplyingpower to a load which operates at a normal current level but is subjectto occasional high current surges, the load having first and secondsides, comprising:a. a current limiting transformer having at least onesecondary winding with first and second terminals, the first terminal inelectrical communication with the first side of the load; and b. asolid-state switch connected in series with the second terminal of thesecondary winding and the other side of the load.
 2. The apparatus ofclaim 1, wherein the current-limiting transformer comprises amagnetic-shunt transformer.
 3. The apparatus of claim 2, wherein theload comprises an incandescent bulb.